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Earth's warming 'extremely likely' due to human activity: report

The panel issuing the report projects average temperatures will rise by 0.5 to 8.6 degrees Fahrenheit by century's end

September 27, 20135:12AM ET

A report by the Intergovernmental Panel on Climate Change suggests global warming is man-made.Jeff Swensen/Getty Images

Scientists can now say with extreme confidence that human activity is the dominant cause of the global warming observed since the 1950s, a new report by an international scientific group said Friday.

Calling man-made warming "extremely likely," the Intergovernmental Panel on Climate Change used the strongest words yet on the issue as it adopted its assessment of the state of the climate system.

In its previous assessment, in 2007, the U.N.-sponsored panel said it was "very likely" that global warming is man-made.

It now says the evidence has grown because of increased and better observations, a clearer understanding of the climate system and improved models to analyze the impact of rising temperatures.

"Our assessment of the science finds that the atmosphere and ocean have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased," said Qin Dahe, a co-chair of the working group that wrote the report.

The full 2,000-page report won't be released until Monday, but a summary for policymakers with the key findings was published Friday. It contained few surprises, since many of the findings had been leaked in advance.

As expected, the IPCC raised its projections of the rise in sea levels to an increase of 10 to 32 inches by the end of the century. The previous report predicted a rise of 7 to 23 inches.

The IPCC assessments are important because they form the scientific basis of U.N. negotiations on a new climate deal. Governments are supposed to finish that agreement in 2015, but it's unclear whether they will commit to the emissions cuts that scientists say will be necessary to keep the temperature below a point where the worst effects of climate change can be avoided.

Using four scenarios with different emissions controls, the report projected that global average temperatures would rise by 0.3 to 4.8 degrees Celsius (0.5 to 8.6 degrees Fahrenheit) by the end of the century.

Only the two lower scenarios, which were based on significant cuts in CO2 emissions, came in below the 3.6-degree Fahrenheit limit that countries have set as their target in climate talks to avoid the worst consequences of warming.

Emissions continue to rise, experts say, mainly because of rapid growth in China and other emerging economies. They say developed countries should take the lead on emissions cuts because they've pumped significant amounts of carbon into the atmosphere longer than other nations have.

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Meyer added that politics has derailed much of the president's climate proposals.

Climate activists said the report should spur governments to action.

"There are few surprises in this report but the increase in the confidence around many observations just validates what we are seeing happening around us," said Samantha Smith of the World Wildlife Fund.

One of the most controversial subjects in the report was how to deal with a purported slowdown in warming in the past 15 years. Climate skeptics say this casts doubt on the scientific consensus on climate change.

Many governments had objections over how the slowing was treated in earlier drafts, and some called for it to be deleted altogether.

In the end, the IPCC made only a brief mention of the issue in the summary for policymakers, stressing that short-term records are sensitive to natural variability and don't in general reflect long-term trends.